1. Field of the Invention
The present invention relates to a focus error detection device, and more specifically to a focus error detection device which is to be used in an optical system for recording and reading out information.
2. Description of Background Information
In a system optically recording and reading out information, such as video disc player or a digital audio disc player, it is common to provide a focus error detection device for automatically adjusting the position of an object lens which is provided for applying a light beam from a light source onto a recording surface of a recording medium. The focus error detection device generally consists of a photo detector for detecting the intensity of light reflected by the recording surface and for producing a signal indicative of an amount and direction of the focus error, and a beam splitter for directing the light reflected by the recording surface to the photo detector. In order to detect the focus error, a cylindrical lens is provided between the beam splitter and the photo detector so that an in-focus position is determined by means of astigmatism of a cylindrical mirror.
However, in the cases of conventional arrangements, there was inevitably a difficulty in that the lateral magnification between a reflecting surface of the recording surface and a receiving surface of the photo detector varies, depending on the direction. Due to the lack of uniformity of the lateral magnification, the output signal of the photo detector becomes asymmetric with respect to the direction of displacement of the object lens from the in-focus position, which has adversely affected the stability of the focus servo system, especially during the lock-in period of the focus servo system. Further, a spurious in-focus position, at which the output signal of the photo detector has a level equal to the level indicative of the in-focus position, has been generated due to the difference of the values of the lateral magnification in different directions.